Electrolytic water splitting into hydrogen and oxygen is a key future technology for regenerative energy storage. While wind, solar photovoltaic or hydro sources produce renewable electricity, it is generally intermittent in nature. In this context hydrogen is a key energy carrier which can be stored either directly in molecular form or by further reversible conversion into hydrogen carrier molecules. The stored molecular energy of hydrogen can later be used for combustion or, more efficiently, in hydrogen fuel cells.
A key challenge in the electrolytic splitting of water is the development of a suitable anode electrode, i.e. an oxygen evolution reaction (OER)-electrode, having a low overpotential, a good stability and a high activity. Specifically, the provision of an OER-electrode having at the same time a suitable activity and stability, i.e. being inert, represents a major technical challenge due to the highly oxidizing conditions at the oxygen evolution electrode.
Ortel, E. et al. (Chem. Mater. (2011), 23, 3201.) describes a proton exchange membrane (PEM) electrolyser technology which makes use of Ti-supported TiO2—IrO2— or TiO2—RuO2-based mixed metal oxide systems as dimensionally stable anodes, while employing Pt as cathode material. However, to allow a wide-spread application at affordable costs, such systems should contain no or a minimal amount of noble metals.
In this respect, transition metal oxides have attracted considerable attention. Gorlin, Y. J. et al. (Am. Chem. Soc. (2010), 132, 13612.) describes a Mn oxide thin film which is electrodeposited onto a polished glassy carbon substrate. Under alkaline conditions, the Mn oxide thin film was found to be active for both, the oxygen reduction reaction (ORR), i.e. the cathode reaction in a fuel cell, and the oxygen evolution reaction (OER), i.e. the anode reaction in an electrolytic cell. However, for industrial purposes a more practical, scalable and inexpensive processes for preparing such an electrode is desired, while still maintaining a high electrolytic activity and stability. Moreover, to allow a wide-spread an environmentally acceptable application of such electrodes it is desirable to provide an electrode which achieves a high electrolytic activity under neutral electrolyte conditions, rather than under alkaline electrolyte conditions.
Jiao, F. et al. (Chem. Commun. (2010), 46 2920.) and WO 2011/094456 A1 describe silica-supported Mn oxide clusters which can be applied as OER-electrode in a neutral solution. However, a suitable OER-electrode must also exhibit a sufficient electronic conductivity in order to minimize ohmic losses and to increase current density during the electrolytic reaction, thus, enabling an economic operation of an electrolytic cell with a reduced overpotential.
WO 2010/027336 A1 describes MnO2 decorated single wall nanotubes being prepared by a symproportionation precipitation method comprising a simple drying step at 100° C. The MnO2 decorated single wall nanotubes are used as an electrode in a supercapacitor comprising an acid polymer electrolyte.
EP 2 140 934 A1 describes a catalyst material comprising aggregates of nanoneedles of manganese dioxide and having a mesoporous structure. The catalyst material is used to oxidatively decompose water under acidic conditions to produce oxygen gas.
Finally, EP 1 831 440 B1 describes a method for preparing a supported catalyst by functionalizing carbon nanotubes with an oxidizing agent to form functionalized carbon nanotubes prior to loading a metal catalyst onto said functionalized carbon nanotubes.
Accordingly, in view of the prior art there is a demand for a process for splitting water under neutral electrolyte conditions, i.e. under environmental acceptable conditions which e.g. allow the splitting of waste water and/or seawater, while achieving a high, and preferably an improved, electrolytic activity and in particular a low overpotential in an oxygen evolution reaction, yet being stable and inert vis-à-vis the oxidizing conditions seen during the oxygen evolution reaction. Moreover, in view of the prior art there is a demand for a process for preparing a material for the splitting of water under neutral conditions, while employing practical, inexpensive and scalable techniques which can be effectively used in an industrial setting.